1. Field of the Invention
The present invention is related to a method capable of controlling brushless DC motor and particularly to a method capable of controlling brushless DC motor with pulse width modulation (PWM) and a device thereof.
2. Brief Description of the Related Art
The brushless DC motor with Hall inductor being instead of the changeable electric brush in the traditional DC motor is much suitable for the micro cooling fan system and widely adopted in the low voltage products such as the computer fan, the copy machine and the Laser printer. The single-phase brushless DC motor usually in the computer fan as shown in FIG. 1 includes a stator 11 with wounded coil sets 10 and a 4-pole rotor 12 for rotating fan blades.
Referring to FIGS. 2 and 3 in company with FIG. 1, the control device 2 of the conventional single-phase brushless DC motor 1 provides a Hall sensor 21 for sensing the magnetic pole positions of the rotor 12 and producing a Hall signal SHA, which is analog signal VHP, VHN, correspondingly. The Hall signal SHA is input to a comparator 22 and converted as a digitalized Hall signal SHD before being sent to a driver 23. A PWM generator 24 produces a PWM signal being sent to the driver 23 based on an external control signal SC (voltage signal). The driver 23 outputs the first PWM signal SPWM1 and the second PWM signal SPWM2 alternately as shown in FIG. 3 corresponding to the magnetic pole positions of the rotor 12 based on the Hall signal SHD and the PWM signal to control the switch circuit 25 switching the phase of the output current IOUT, which is sent to coil sets 10, alternately for rotating the rotor 12.
However, instantaneous change during switching between the first PWM signal SPWM1 and the second PWM signal SPWM2 causes the output current IOUT to produce sharp wave IP easily and it results in mechanical consonance and noise.